The invention relates to a method and a device for digital-to-analog conversion of a signal.
Apart from fast analog-to-digital converters, there is also a need for fast digital-to-analog converters for digital signal processing. Such devices or circuit arrangements for digital-to-analog conversion (called D/A conversion in the following) are used for digital signal processing in, for example, television, radio broadcasting or radio receiving technology, as D/A changers, D/A converters or D/A transformers for image signals and sound signals. In that case digital signals are, for processing, converted into analog signals.
The performance capability of digital signal processing has expanded through constantly increasing capacity of memory chips as well as increasing performance of processors at great speed. The performance capability of D/A converters with respect to resolution and bandwidth has increased substantially more slowly by comparison with components of that kind in digital signal processing. In particular, fast D/A converters are needed for direct digital frequency synthesis (abbreviated to DDS), since the performance of the fastest DDS modules currently available is limited by the D/A converter.
The invention therefore has the object of providing a method and a device for digital-to-analog conversion of a digital signal in which an improved performance capability may be possible with respect to bandwidth and resolution capability.
According to a first aspect of the invention there is provided a method of digital-to-analog conversion of a band-limited digital signal, in which the signal is transformed on the basis of orthogonal functions, wherein coefficients associated with the orthogonal functions and the signal are determined and these are subjected to digital-to-analog conversion and wherein the signal is transformed back in the analog region on the basis of the analog coefficients, which result therefrom, by means of orthogonal functions.
Such a method proceeds from the consideration that, instead of sequential digital-to-analog conversion of individual scanning values of a conventional D/A converter, a whole interval of the time function of the signal is processed. For that purpose, the signal time-limited to the interval is preferably described on the basis of orthogonal functions. The signal is preferably broken down into several intervals. Through limitation of the time function of the signal to the interval with subsequent transformation by means of orthogonal functions, the signal is fully determined in the digital region on the basis of digital coefficients of the orthogonal functions in equidistant or non-equidistant spacing and can be reconstructed from these coefficients. In other words, the digital signal is processed on the basis of orthogonal functions into an equation for its transform, which is then converted from digital to analog and transformed back in the original region, whereby the original function of the signal is determined in the analog region.
Expediently, the signal is limited in the time region to the interval and is represented within the interval by a sum of orthogonal functions with a presettable number of summands, wherein the coefficients, which are associated with the orthogonal functions, for the interval are determined and subjected to digital-to-analog conversion and wherein the signal is represented in the analog region by multiplication of the analog coefficients, which result therefrom, by orthogonal functions. The signal is preferably resolved into several intervals so that the signal can be represented over a large time range. In the case of band limitation of the signal, the scanning theorems are preferably followed. According to the scanning theorems, discrete values of the frequency function or time function suffice for complete description of the signal in the case of limitation of the time function or frequency function. The time function of the signal is preferably represented by development according to a complete system of orthogonal functions. The band-limited signal is fully described by a finite summation.
The achievable quality of the approximation results from the number of summands, which is discontinued in a real system after a finite number. In that case, the minimum value for the number N of the summands (also termed support points) results from the scanning theorems in the time region and frequency region for time-limited and band-limited signals. The number of summands N is preferably determined by the equation:                     N        =                  T          τ                                    (        1        )            
wherein T=length of the interval in the time region and xcfx84=segment in the time region,
wherein                     τ        =                              1                          2              ⁢              B                                ⁢                      xe2x80x83                    ⁢                      (                          Nyquist              ⁢                              xe2x80x83                            ⁢              criterion                        )                                              (        2        )            
wherein B=bandwidth.
The number of summands is in that case preferably selected so that a sufficient resolution is ensured. The systems of orthogonal functions in the digital region (transformation) and in the analog region (inverse transformation) are preferably selected to be the same. Alternatively, the systems of orthogonal functions (also termed basic functions) can also be different.
Expediently, the digital signals are transformed in such a manner that these are multiplied in the digital region by presettable orthogonal functions and the digital coefficients associated with these functions are ascertained. The digital signal is fully described in the digital region on the basis of this transformation. In the example of Walsh functions, the transformation (=determination of the inner product) is described in accordance with the following equations:                                                         x              d                        ⁡                          (                              t                i                            )                                =                                                                      ∑                  j                                N                            ⁢                                                a                  j                  d                                ·                                                      g                    j                    d                                    ⁡                                      (                    t                    )                                                                        =                                                            ∑                  j                                N                            ⁢                                                (                                                                                    x                        d                                            ⁡                                              (                        t                        )                                                              ,                                                                  g                        j                        d                                            ⁡                                              (                        t                        )                                                                              )                                ·                                                      g                    j                    d                                    ⁡                                      (                    t                    )                                                                                      ,                            (        3        )            xe2x80x83adj=xcexa3xd(ti)xc2x7wal(j"PHgr")xc2x7xcfx84xe2x80x83xe2x80x83(4)
wherein   Θ  =      t    T  
and, for example, wherein gjd(t)=wal (j,xcex8)=Walsh functions, wherein xd(t)=time function of the digital signal, gjd(t)=orthogonal functions in the digital region, ajd=coefficients, in the digital region and N=number of summands (=number of parallel channels or branches or D/A converter).
The equation (3) is the definition of the inner product between xd(ti) and gjd(t). For brevity, the symbolic term (x(t), gj(t)) is used in the following.
In a case where basic functions differ in the digital and the analog, the linking of the coefficients takes place by a linear transformation according to:                                           x            ⁡                          (              t              )                                =                                                                      ∑                  j                                N                            ⁢                                                a                  j                  d                                ·                                                      g                    j                    d                                    ⁡                                      (                    t                    )                                                                        =                                                            ∑                  j                                N                            ⁢                                                b                  j                                ·                                                      h                    j                                    ⁡                                      (                    t                    )                                                                                      ,                            (        5        )            
under the precondition that gj(t)≈hj(t), wherein x(t)=time function of the signal, gjd(t)=orthogonal functions in the digital region, aj, bj=coefficients in the analog region, hj(t)=orthogonal functions in the analog region, ajd=coefficients in the digital region and N=number of the summands.
For determination of the coefficients bj in equation (5), the scalar product (inner product) is formed.                                           ∑            j                    ⁢                                                    (                                  x                  ,                                      g                    j                                                  )                                            ⏞                                  a                  j                  d                                                      ⁢                          g              j                                      =                                            ∑              j                        ⁢                                                            (                                      x                    ,                                          h                      j                                                        )                                                  ⏞                                      b                    j                                                              ⁢                              h                j                                              |                                    h              i                        ⁢                          xe2x80x83                        ⁢            formation            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            inner            ⁢                          xe2x80x83                        ⁢            product                                              (        6        )                                                      ∑            j                    ⁢                                    (                              x                ,                                  g                  j                                            )                        ⁢                          (                                                g                  j                                ,                                  h                  i                                            )                                      =                  (                                    x              ,                        ⁢                          h              i                                )                                    (        7        )                                                      ∑            j                    ⁢                                    a              j                        ⁡                          (                                                g                  j                                ,                                  h                  i                                            )                                      =                  b          i                                    (        8        )            
In that case, the coefficients in the digital region are preferably ascertained on the basis of a transformation matrix with matrix elements (gj, hi)=mj,i according to:                               (                                                    …                                                                    …                                                                                      b                  i                                                                                    …                                              )                =                              (                                                            …                                                  …                                                  …                                                  …                                                                              …                                                  …                                                  …                                                  …                                                                              …                                                  …                                                  …                                                  …                                                                              …                                                  …                                                  …                                                                      (                                                                  g                        j                                            ,                                              h                        i                                                              )                                                                        )                    ⁢                      xe2x80x83                    ⁢                      (                                                            …                                                  …                                                                      a                    j                                                                    …                                                      )                                              (        9        )            
In accordance with the respective prescriptions and criteria for the digital signal processing, trigonometric functions, Walsh functions and/or complex exponential functions are used as orthogonal functions. In the analog region, trigonometric functions, for example sine and or cosine functions, are preferably used. In the digital region, functions such as, for example, Walsh or Haar functions are preferably used, which functions can assume only the value +1 or xe2x88x921.
Preferably, the analog coefficients are transformed back in such a manner that the signal is described in the analog region by multiplication of the analog coefficients by orthogonal functions and subsequent summation. For example, the signal is represented in the analog region on the basis of the generalised Fourier analysis:                                           x            ⁡                          (              t              )                                =                                                                      ∑                  j                                N                            ⁢                                                a                  j                                ·                                                      g                    j                                    ⁡                                      (                    t                    )                                                                        =                                                            ∑                  j                                N                            ⁢                                                (                                                            x                      ⁡                                              (                        t                        )                                                              ,                                                                  g                        j                                            ⁡                                              (                        t                        )                                                                              )                                ·                                                      g                    j                                    ⁡                                      (                    t                    )                                                                                      ,                            (        10        )            
wherein x(t)=time function of the signal, gj(t)=orthogonal functions, aj=coefficients, N=number of the summands=number of the orthogonal functions=number of the support points in the transformed region (frequency region for the special case of the Fourier transformation)=number of the parallel channels and T=length of the interval in the time region.
In an orthonomised system there applies for the inner product of orthogonal functions:       (                  g        j            ,              g        i              )    =      {                                                                      0                ,                                                                                      when                  ⁢                                      xe2x80x83                                    ⁢                  j                                ≠                i                                                                                        1                ,                                                                                      when                  ⁢                                      xe2x80x83                                    ⁢                  j                                =                i                                                    ⁢                  xe2x80x83                ⁢                  (                                    h              j                        ,                          h              i                                )                    =              {                                                            0                ,                                                                                      when                  ⁢                                      xe2x80x83                                    ⁢                  j                                ≠                i                                                                                        1                ,                                                                                      when                  ⁢                                      xe2x80x83                                    ⁢                  j                                =                i                                                        
According to a second aspect of the invention there is provided a device for digital-to-analog conversion of a band-limited digital signal with an input module for transformation of the signal in the digital region by means of orthogonal functions, a module for digital-to-analog conversion of digital coefficients of the transformation function and an output module for transformation back of the signal in the analog region.
Advantageously, the input module serves for representation of the signal within the interval by a sum of orthogonal functions with a presettable number of summands. For preference, the entire signal is broken down into several intervals. Preferably, the determination of the coefficients for the interval takes place by means of the input module.
The digital signal is described within the interval by means of the input module on the basis of the orthogonal functions. Subsequently, the digital coefficients associated with the functions are determined, wherein the digital-to-analog conversion of the N coefficients takes place in N modules, for example in N conventional D/A converters. The signal can be fully represented in the analog region through multiplication of the analog coefficients, which are thus ascertained, by the orthogonal functions in the analog region by means of the output module and subsequent summation.
In an advantageous embodiment the input module comprises a number, which corresponds with the number of summands, of cells of a shift register and a corresponding number of N multiplicators and summation elements. The shift register and the multiplicators serve for the transformation of the digital signal on the basis of presettable orthogonal functions. The digital coefficients associated with the functions are ascertainable by means of the summation elements. A particularly simple construction of the device, in terms of circuitry, for transformation of the digital signal thereby results.
After the transformation of the digital signal and consequently the determination of the coefficients of the orthogonal functions in the digital region, the digital-to-analog conversion of the coefficients can be carried out by means of the conventional D/A converters. In the analog region the output module advantageously comprises a number, which corresponds with the number of summands, of multipliers and a summation element. The multiplier serves for multiplication of the respective analog coefficients by the orthogonal functions in the analog region. Through subsequent summation of all parallel branches, the signal can be fully represented in the analog region. The number of branches or channels in that case corresponds with the number of summands. The device can comprise N branches with N cells of the shift register, N times the number of multiplicators, N summation elements, N D/A converters and, for the transformation back at the output side, N multipliers and the summation element. A device, which has this construction in terms of circuitry, for digital-to-analog conversion of the signal is also termed a correlation digital-to-analog converter.
In addition, an integrator, for example a low-pass filter, can be provided. The low-pass filter is preferably connected downstream of the summation element for smoothing the functions in the analog region.
Advantageously, the above-described device for digital-to-analog conversion of a signal comprises a direct digital frequency synthesis module (DDS module). The performance of the DDS module is substantially improved relative to conventional DDS modules in the respect of a particularly high scanning rate.
The advantages achieved by a method exemplifying and a device embodying the invention are that by comparison With a single conventional D/A converter with a high scanning rate, the scanning rate of the individual D/A converter of the device can, through the plurality of parallelly connected D/A converters (number of parallel branches equal to number of presettable summands), be selected to be smaller by the factor of the number of summands.